The invention refers to an apparatus to measure physical and/or chemical properties, especially for temperature measurements.
From DE 199 60 370 an apparatus for temperature measurement is known that utilizes an optical resonator. The optical resonator is shaped as microparticle. Light of a broadband light source (a laser diode) is coupled into the microparticle via thin light guiding tapered fibers, and decoupled from the microparticle and transferred to an optical spectrometer for evaluation. Within the spherical microparticle, light experiences multiple reflections due to total reflection on the surface. The wave trains interfere and—at certain wavelength of the light—resonances appear (the amplitude of the electromagnetic field in the microparticle is greatly enhanced). The resonance properties of the optical resonator depend on temperature due to thermal expansion and the change in index of refraction. The decoupled light is monitored by an optical spectrometer and the resonance spectrum is converted into a temperature value.
The method for temperature measurement described in DE 199 60 370 has a great number of advantages. Due to the small size of the resonator, it is suitable for temperature measurements with high spatial resolution. Because the measuring method is purely optical, the sensor can be used in environments with explosion risk as well as in environments with strong electromagnetic fields (e.g. nuclear spin tomography, microwave oven etc.).
In the case of the sensor described in DE 199 60 370 the coupling and decoupling of light is achieved by flexible optical fibers, with the fibers shaped into tips tapering down to just a few microns. These tips are connected to the microparticle by a photopolymeric adhesive.
In practical use, this sensor embodiment has some problems. Requirements on the photopolymeric adhesive are substantial. Due to the necessary exact positioning and mechanical fixing, the manufacturing of the sensor is complex.